U.S. Pat. Nos. 4,781,546 and 4,836,757 to Curwen, both of which are hereby incorporated by reference, disclose a conventional free piston-type compressor having a piston reciprocally fitted within a cylinder. An electromagnetic motor reciprocates the piston within the cylinder, and as the piston reciprocates, working fluid is compressed on both sides of the piston. A piston rod extending through the compression chamber has one of its ends connected to the piston member. A balance chamber is disposed opposite to the compression chamber relative to the piston. As the piston reciprocates, its inertia is balanced by the recoil strength of a coil spring acting on one side thereof and by the compression load of the balance chamber acting on the other side thereof.
Since the compression chamber is disposed on the side of the piston chamber through which the piston rod extends, there is a resulting reduction in the total volume of the compression chamber. Consequently, the volumetric efficiency of the compressor is reduced. Moreover, the high pressure in the compression chamber must be contained and sealed by, among other things, a seal disposed around the reciprocating piston rod. Since the seal abuts the piston rod, and is, therefore, subjected to reciprocating movement, it must be suitably designed to withstand a relatively harsh environment in addition to the high pressure of the compression chamber.
Furthermore, the arrangement of the valve mechanism, which includes a valve plate, suction and discharge holes, and suction and discharge valves, must accommodate the piston rod extending therethrough. Accordingly, the arrangement of the valve mechanism is limited.